285s - MPG dropped... Normal?

[Supercharged111]

Get a tune, the truck will stop hunting for a gear and hold OD much better. Can also remove the lip on the throttle blade. And do header back exhaust. The heavier tires hurt my mpg too, but I'm not going back to stock. Your speedo was probably 5 fast before the tires and you were likely never getting the 15 you thought you were. My truck lied from day 1 about the speed and odometer. I corrected them with GPS.

 

[98chevy2500SS]

If I had to guess it would be at least $100. They seem to want that much at a minimum to just plug in there diagnostic tool.

I guess it’s possible that the transmission shop recalibrated the speedometer. Did you have it rebuilt or did the previous owner? If you did maybe that transmission shop would do it for you for less than the dealer.

The previous owner got the tranny rebuilt 3-4 years ago, I thought, the guy gave me the papers when I bought it, which was good. $100 doesn't seem too bad, a lot less than what I was thinking, I was thinking they would charge at least $250-300 for it. I'll give my local dealer a call this week about it and see what they can do for both trucks.

I noticed quite a difference when I went to 285's from 265's myself. In my situation there wasn't a tremendous difference in height, but a significant difference in weight, wider tread, more rolling resistance, effectively lower rear ratio, all added up to more throttle to get it and keep it rolling. On my truck there is also a huge difference in consumption between putting along 65-68 down the highway or running 75. On my regular 12 hr round trips 5-7 mph faster works out to be almost 2 tankfulls round trip.

I like the 285's with the 3.73's towing in 3rd puts me better in the torque curve about 2.6k rpms running about 62-65mph and it actually doesn't do bad mileage wise towing 4k lbs.

That makes sense, with the wider tread, weight, and more rolling resistance. I'm probably just going to keep my 285s until I have to replace them, and maybe then I'll have more money to wake the truck up to handle 285s better.

Get a tune, the truck will stop hunting for a gear and hold OD much better. Can also remove the lip on the throttle blade. And do header back exhaust. The heavier tires hurt my mpg too, but I'm not going back to stock. Your speedo was probably 5 fast before the tires and you were likely never getting the 15 you thought you were. My truck lied from day 1 about the speed and odometer. I corrected them with GPS.

lol, my 98 is probably lying to me as well, I thought 15 mpg wasn't right, it's probably getting 12 or something. It reads 15 over. I have thought of sending both computers into Black Bear and see what they can do. I have thought about getting headers for both trucks, this fall when the swap meet comes, I'll have to be on the lookout for some stainless ones.

 

[Supercharged111]

Just buy a spare computer and ship it off so you don't have any down time. I don't know if BB can do 96-97.

 

[kennythewelder]

BB will ship you an ECM that is programed and ready for install. You get your core charge back when they get your ECM. And yes BB can do a 96 up for these trucks.

 

[GreenZ98]

Ive got a blackbear tuned computer and had them recalibrate it for 285s. I usually get between 12-14 mpg.

 

[98chevy2500SS]

Checked out Black Bear and I can get mine tuned for $305 with ECM deposit. Then I saw this description below:

Due to the difficulty in maintaining satisfactory inventory levels on your computer type, we encourage any customer that is able to send a computer in to be tuned to do so rather than utilize a ECM deposit for the quickest turnaround time.

So with an ECM deposit, am I getting a new computer without having to send in my original computer? Or is there a core charge?

 

[kennythewelder]

Yes, and yes. You are getting an ECM that has been tuned to your specs from there inventory. Then you send in your ECM and get your core charge back. $50 for an ECM that has peeling paint, or $100 for one that has 95 percent of its paint left on it, something like that. Then what they do is tune your ECM and sale it to someone else, unless you pull your ECM,send it to them, waite for them to tune it, and send it back to you. Or if you go to one of there live events, then you can have your truck tuned on sight. Its all on there web sight.

 

[df2x4]

Just buy a spare computer and ship it off so you don't have any down time. I don't know if BB can do 96-97.

They can, they tuned my red '97. They'll supply another PCM too if you pay the deposit so there's no down time.

Checked out Black Bear and I can get mine tuned for $305 with ECM deposit. Then I saw this description below:

So with an ECM deposit, am I getting a new computer without having to send in my original computer? Or is there a core charge?

As mentioned, if you pay the deposit they'll send you a tuned PCM and you can either keep your stock one or send it back to them for a deposit refund. I chose to keep mine and later had it tuned for a different octane level so I can switch them out if I want.

 

[bugdewde]

285s are about an inch taller and will cover more distance per revolution of the tire. The Speedo & odometer will be about 3.8 % slower or behind.

See this "tire size comparison" site. Provides visual & numerical comparisons.
https://tiresize.com/comparison/

Calculate speed on the fly?
So basically, just multiply your indicated speed by 4% (round up 3.8 for easier math) to get a number to add to your speed.
50 mph x .04 = 2.0 ..... 50 + 2 = 52 mph
Been doing this for decades with my lifted Jeeps (an easy 10% calculation)

Same for your odometer. 250 miles is actually close to 260.
250 x .04 = 10.
250 + 10 = 260.

Eventually, you'll do it in your head without thinking too much about it.
60 mph? 6x4=24. Add 2.4 = 62.4 mph.
70 mph? 7x4=28. Add 2.8 = 72.8
80 mph? 8x4=32. 83.2

Screenshot from the site I listed above:

You must be registered for see images attach

Most all tire diameter & width increase will cost mileage. While it may lower rpms at a given speed, it requires more fuel to keep the extra weight rolling at the same speed.
..... The killer is the added wind resistance of the extra width (tire surface area) as well as increased area for air to go under the vehicle (very un-aero dynamic under there) causing more resistance.... requiring more throttle/fuel.

There's multiple force vectors at work in the physics of increased tire size.

 

[GreenZ98]

Checked out Black Bear and I can get mine tuned for $305 with ECM deposit. Then I saw this description below:

So with an ECM deposit, am I getting a new computer without having to send in my original computer? Or is there a core charge?

I went the same route as df2x4 and kept my computer just to have as a spare.